1. Field of the Invention
The invention relates generally to encrypting and decrypting digital transmissions. More specifically, the invention relates to a method and apparatus for recording and retrieving encrypted digital data.
2. Background Art
Transmission of encrypted data is well-known in the field of pay TV systems, where scrambled audiovisual information is broadcast typically by satellite to a number of subscribers, each subscriber possessing a decoder or receiver/decoder capable of descrambling the transmitted program for subsequent viewing.
In a typical system, scrambled digital data is transmitted together with a control word for descrambling the digital data, the control word itself being encrypted by a first key and transmitted in encrypted form, the scrambled digital data and encrypted code word being received by a decoder possessing an equivalent of the first key necessary to decrypt the encrypted code word and thereafter descramble the transmitted data. A paid-up subscriber will receive on a monthly basis the key necessary to decrypt the encrypted control word so as to permit viewing of a particular program.
With the advent of digital technology, the quality of the transmitted data has substantially increased. A particular problem associated with digital quality data lies in its ease of reproduction. Where a descrambled program is passed via an analog link for viewing and recording by a standard VCR, the quality is typically no greater than that associated with a standard analog cassette recording. The risk that such a recording may be used as a master tape to make pirate copies is thus no greater than with a standard purchased analog cassette recording.
In contrast, any descrambled digital data passed via a direct digital link to a new generation digital recording device (for example, a DVHS recorder) will be of the same quality as the originally transmitted program and may thus be reproduced any number of times without any degradation of image or sound quality. There is, therefore, a considerable risk that the descrambled data will be used as a master recording to make pirate copies, including additional digital copies or analog VHS copies.
French Patent Application 95 03859 shows one method of overcoming this problem with a system in which descrambled digital data is never allowed to be recorded on the digital recording medium. Instead, the decoder described in this application records the data in its scrambled form on the recording medium together with the control word necessary to descramble the data re-encrypted by means of another key. This new key is known only to the receiver/decoder and replaces the first key needed to obtain the code word for viewing the program.
One advantage of such a system is that the data is never stored in a clear form and cannot be viewed without possession of the new key, which is stored in the receiver/decoder. The system also has the advantage that, because the first key changes on a monthly basis, the use of a constant key to re-encrypt the control word registered on the digital tape means that the receiver/decoder will still be able to decrypt the control word recorded on the tape even after the end of a subscription month.